Measuring Electricity
Ammeters
- An ammeter is the device used to measure electric currents.
- It has to be connected in series with the other electrical components in the circuit.
- By connecting in series, the current will flow into the ammeter by the positive (red) terminal and leaves by the negative (black) terminal.
- The deflection of the pointer of the ammeter shows the amount of current flowing through the circuit.
- The SI unit for electric current is the ampere (A).
- If it is connected wrongly, the ammeter
- may suffer from damages due to wrong connections
- does not measure the current of the required component
- does not show the correct reading as the pointer moves the wrong way.
- Figure X shows the correct and incorrect ways
- To measure the correct amount of current flowing in a circuit, an ideal ammeter should have a very low resistance so that the amount of electrical energy wasted is very small.
Voltmeters
- A voltmeter is the device used to measure the voltage across an electrical component.
- It has to be connected in parallel across the other electrical components in the circuit.
- A negligible amount of current enters the voltmeter by the positive (red) terminal and leaves by the negative (black) terminal.
- The deflection of the pointer of the voltmeter shows the voltage across the component.
- The SI unit for voltage is the volt (V).
- If it is connected wrongly, the voltmeter
- does not measure the voltage of the required component.
- does not show the correct reading as the pointer moves the wrong way.
- To avoid drawing a large amount of current from the circuit, an ideal voltmeter should have a resistance much larger than the component it is connected across.
Measuring the Current and Voltage of an Electric Circuit
- Set up the circuit as shown in Figure X.
- 'i' indicates the current flow in the circuit.
- The ammeter will show the amount of current flowing through the circuit.
- The voltmeter will show the voltage across the resistor R.
- By measuring the current and voltage, we can obtain the resistance of resistor R.
- The SI unit for resistance is the ohm (Ω).
